This invention relates to an air conditioning system particularly adapted for use in an automotive vehicle, and more particularly to a control system for an air conditioning system of this kind, which is adapted to control both the capacity of the compressor and the quantity of air being supplied to the evaporator so as to maintain the evaporator boiling pressure within a desired range.
Air conditioning systems for automotive vehicles include a type equipped with a control system for controlling the refrigeration capacity of the air conditioning system in response to heat loads thereon. One of such control systems is known from British patent specification No. 1,587,111, which controls the evaporator boiling pressure by regulating the capacity of a variable delivery compressor in response to the difference between a detected value of the compartment temperature and a preset value thereof. Further, the British patent specification points out a problem that a too high evaporator boiling pressure causes a degradation in the dehumidifying capacity to thereby impede a required dehumidifying action, whereas a too low evaporator boiling pressure causes freeze-up of the evaporator, and it provides a solution to this problem, which comprises detecting the evaporator boiling pressure to control the same pressure to values within a predetermined or desired range. According to this proposed control system, in order to control the evaporator boiling pressure, the capacity of the compressor alone is varied in response to the heat load requirements while maintaining the flow rate of air passing over the evaporator at a certain value. However, the range of capacity of the compressor is limited that can be controlled for ensuring required dehumidifying capacity of the evaporator and prevention of freeze-up of the evaporator. Therefore, if the evaporator boiling pressure lies outside the controllable range of the compressor capacity, there will take place overcooling or poor cooling.